fixed element-wise-divide by zero warnings, some refactoring
fixed element-wise-divide by zero warnings, some refactoring

*  results of the element-wise divide-by-zero are now replaced with 0 as per  in the conversion of XYZ to LUV
* single-character, capitalized variable names are okay since there's so much math here

TadLeonard/husl-numpy,TadLeonard/husl-numpy,TadLeonard/husl-numpy